The invention relates to a method for the compression and decompression of image data.
Today there are a plurality of technologies and compression methods for storing image data. The technologies used derive essentially from pixel-based and vector-based methods.
In the case of the pixel-based technologies a raster is laid over the image. Each raster point corresponds to one pixel. A pixel value is encoded with one bit (for example, WBMP→black and white) or several bits (for example, BMP→true color). In order to reduce the size of the image files various compression methods can be applied. An effective method consists of reducing the bits per pixel. Furthermore, methods can be applied in which pixels are combined into groups. These are, as a rule, square areas which are then transformed with the aid of the DCT (discrete cosine transformation) into the frequency domain. The high frequency content arising in the transformation can be ignored without noticeable loss of quality. This method is, for example, applied in JPEG images. In the case of very high JPEG compressions it can lead to distorting, disordered patches of rectangles in the image, so-called artifacts.
In the case of vector-based methods the image is described unambiguously with the aid of geometric forms (for example, rectangles, or circles). A known method is the so-called SVG (scaleable vector graphics). This method can be used with outstanding results in technical drawings since images prepared in this manner can be scaled very well. The corresponding files are clearly smaller than in traditional binary bitmap formats. However, these methods are less suited to storing photographs.
Along with the traditional photographic apparatuses which expose a film, more and more there are photographic apparatuses which store the photograph digitally on a storage medium instead of on film. Since these storage media only have a limited capacity, only a small number of high-resolution photographs, or many images with low resolution, can be stored. A flexible scaling is not possible at the present time.